The invention relates to a process for preparing isophorone (3,5,5-trimethyl-2-cyclohexen-1-one).
The invention especially relates to an extension of the process for preparing isophorone for improved raw material utilization. This involves rehydrolysing the organic residues obtained in order to recycle the materials of value which form into the process. A “material of value” is understood to mean, as well as acetone and isophorone, especially also the intermediates phorone, mesityl oxide and diacetone alcohol.
Isophorone is used inter alia as a high-boiling solvent in the coatings industry, printing inks industry, adhesives industry and crop protection industry. In addition, isophorone may be processed further, for example to give isophoronenitrile, isophoronediamine, isophorone diisocyanate or ketoisophorone.
Isophorone is the trimeric condensation product of acetone. Isophorone is prepared via a catalysed aldol condensation of acetone.
The reaction in the liquid phase is described in the patent literature virtually exclusively under alkaline conditions at elevated temperatures and high pressures.
The patent documents of Hibernia Chemie (DE 10 95 818, DE 11 44 269, DE 12 05 525, DE 11 65 018) from the 1960s describe not only the use of a monophasic reactant/catalyst mixture with low alkali concentrations but also workup by means of a hydrolysis column. According to these documents, isophorone is prepared in a pressure reactor by condensation of acetone in the liquid phase by means of amounts of alkali (NaOH or KOH) of less than 1% as a catalyst and using amounts of water of less than 20% at temperatures of 150-250° C. The two phases which form in the reaction are emulsified both by a suitable reaction regime (reactor construction, pulse generator) and by the use of an emulsifier in order to achieve good contact between catalyst and the reactants (DE 10 95 818).
In addition, DE 12 05 525 describes the workup of by-products, called overcondensates. At 120-300° C., the hydrolysis of the overcondensates takes place with an aqueous alkali solution in what is called a pressure distillation column with constant removal of the acetone formed.
Pure isophorone is recovered from isophorone-containing condensation products by a removal of the low boilers by distillation under the same pressure at which the condensation is performed and by a further workup of the overcondensates still existing by distillation under reduced pressure (DE 11 44 269).
According to the application by BP Chemicals, use of potassium hydroxide solution (KOH) instead of the otherwise customary catalyst, sodium hydroxide solution (NaOH), can enhance the isophorone yield by up to 7% with constant selectivity (DE 25 20 681).
It has also been stated that the product quality of the isophorone can be increased by discharging colouring substances from the reaction column in a sidestream, and purifying this stream by distillation and acidic reaction (DE 26 45 281).
There also exist applications regarding isophorone preparation from Daicel Chemical Industries (JP 8245485, JP 8245486) from the 1990s. These state that reduction of the water concentration in the reactant stream, and also recycling of the aqueous alkali phase after phase separation into the hydrolysis section of the reactive distillation, can enhance the isophorone conversion.
WO 2012/076314 discloses a process for preparing isophorone by catalysed aldol condensations with acetone as a reactant, workup of the reaction product, hydrolysis of the stream of value and separation into an organic fraction and an aqueous fraction, recovery of isophorone from the organic fraction, distillative workup of the aqueous fraction and passage of the vapours from the top of the distillative workup apparatus onward into the hydrolysis apparatus.
In this process, the redissociation of the overcondensates in the hydrolysis does not take place completely because of the equilibrium positions of the reactions that take place, and so overcondensates that are still hydrolysable, which can be converted to materials of value by an additional hydrolysis, are present in the residues obtained at the end of the isophorone purification. In the existing process according to WO 2012/076314, these residues are sent to incineration.
The synthesis of isophorone forms a whole series of by-products. These are, for example, diacetone alcohol, mesityl oxide, phorone, mesitylene and a series of higher condensation products (overcondensates) of acetone (e.g. xylitones and isoxylitones). For this reason, the achievement of high yields and selectivities for isophorone is difficult to attain.
The technical problem addressed by this invention was therefore to find a process which enables an increase in the economic viability of isophorone preparation.